Better, faster, cheaper: Doing business with the sun

August 12, 2011

A solar cell is treated with a laser before depositing the contacts. Credit: Fraunhofer ILT

The change in energy policy has been decided; Germany needs more green energy. From Sep. 5-9 in Hamburg, everything will revolve around our biggest energy supplier: the sun. At the European Photovoltaic Solar Energy Conference, in Hall B4G, Stand C12, Fraunhofer researchers will present new methods for making solar cells cheaper and more efficient.

These days, black panels can be seen on many building roofs, particularly in Southern Germany. Many of these solar collectors are used to heat water, but increasingly there are also photovoltaic systems that directly convert the light of the sun into electrical current. To date, however, only some 2% of electrical current in Germany comes from solar energy, because solar cells are costly and complicated, particularly in production. Researchers from the Fraunhofer-Gesellschaft are developing innovative production methods to change this. Lasers in particular create whole new potentials for production. Dr. Malte Schulz-Ruhtenberg of the Fraunhofer Institute for Laser Technology ILT explains the main advantage: "Laser technology permits contact-free, precise and quick processing." As a result, better solar cells can be produced at lower cost.

Laser Methods in Demand

One example is high-rate laser drilling, which creates tiny holes in solar cells very precisely and quickly. Why? A classic solar cell generates current through the photoelectric effect. It consists of several conducting and semiconducting layers. When light falls onto the cell, negative charge carriers are released from their bonds, and electrical current flows as a result. To date, the contacts for drawing away the electric current generated have been positioned at the front and rear of the cell. Moving all of the contacts to the rear, where they do not cast a shadow, increases the level of energy generated. The holes pave the way for this approach, which is known as "emitter wrap-through", or EWT for short.

Special polygon scanners can be utilized to provide for even higher speeds and higher throughput rates. With these laser scanners, rotating polygonal mirrors precisely deflect millions of laser pulses per second. This allows them to process large areas very quickly. "This is a promising technology that can be used for many laser processes," Dr. Schulz-Ruhtenberg points out.

Precise and gentle on the Material

Aside from speed, the possibility to control all properties of the laser light also plays a major role in solar technology; after all, the cells and wafers  the basic elements of a solar cell  are sensitive. Laser beams can be so finely dosed and controlled that they place nearly no strain on the cells. This is why the Fraunhofer-researchers use them for nearly everything: to drill, melt, cut or solder.

Ultra-short-pulse lasers are used, for instance, to insulate a solar cell's front and back sides from one another. These lasers are gentler than other methods, and that is important, since a large portion of the costs involved owes to damage and breakage during production.

Testing automation systems

Oftentimes, damage is caused by the different kinds of handling devices that manufacturers use in their production environments. They are designed to be as quick and accurate as possible but without damaging the sensitive parts. This reduces the costs involved. At the Fraunhofer Institute for Manufacturing Engineering and Automation IPA, researchers are working to improve the automated handling of wafers and solar cells. "In our test and demonstration center, we are trying to physically simulate handling and automation in photovoltaics and based on this optimizing it," explains Roland Wertz, the project manager responsible at IPA.

This facility provides an interface between industrial production and research service in the area of automation. All of the factors and parameters are recorded under highly-realistic conditions, including factors that influence the precision and speed of various gripper systems. They are assisted in this by the robot ABB IRB 360, also known as FlexPicker, which can also be seen at the Fraunhofer stand. To conduct experiments it is used as a manipulator which can be equipped with various gripping devices that are based on different functional principles. This enables the scientists to analyze and assess products made by various manufacturers and in a standardized way. After all, each specific application has its own requirements and calls for optimized handling.

Less Is More

Savings and improvements are not limited to the production process; they are also directed at the materials used. No more than absolutely necessary - that is the principle behind thin-film solar cells. These usually consist of an inexpensive substrate to which the electrically active material is applied in the form of an ultra-thin film. To be able to produce thin-layer solar cells that are of high quality and economically to make, the Fraunhofer Institute for Surface Engineering and Thin Films IST has developed various processes that improve each and every step of production.

For instance, the semi-conductor layers, the heart of the solar cell, are produced using the hot wire CVD process. "One benefit over conventional methods is the gentle form of coating production," explains Dr. Volker Sittinger of IST. In conventional, plasma-activated CVD, during the coating process the material is bombarded with high-energy particles. The hot wire CVD approach is different: there, the gases that create the film are not activated in plasma but on hot wires. The result is a gentle approach to creating films of high quality. Better use can also be made of the silane gas required in production. "With the hot wire CVD method, we convert up to 90% of the gas used to film material", Sittinger adds.

Recently developed for the contact layers on the front and rear of the cell is the C² coating technology (cylindrical magnetron co-sputtering). With this technology, the material composition can be varied during the coating process. And there are plans to thin things down even further. Coatings only a few nanometers thick are expected with a new type of 3-dimensional solar cell design. The only way to achieve this is with a precision-contour precipitation of the layers, but there is a method to accomplish this: ALD, which stands for atomic layer deposition, stems from the field of nanotechnology.There is thus no need for solar cells to remain unaffordable expensive. New technologies could propel solar energy a major step forward.

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